As an ion exchange membrane to be used for alkali chloride electrolysis which comprises electrolyzing an aqueous alkali chloride solution such as seawater and thereby producing an alkali hydroxide and chlorine, an electrolyte membrane made of a fluoropolymer having ion exchange groups (such as carboxylic acid type functional groups or sulfonic acid type functional groups) has been known.
As the electrolyte membrane, a reinforced electrolyte membrane reinforced by a woven fabric made of a reinforcing thread (such as a thread of polytetrafluoroethylene (hereinafter referred to as PTFE)) is usually used with a view to maintaining mechanical strength or dimension stability. However, the reinforced electrolyte membrane having an electrolyte membrane reinforced by a woven fabric made of e.g. a thread of PTFE tends to have a high membrane resistance, whereby the electrolysis voltage tends to increase.
In order to suppress the membrane resistance of the reinforced electrolyte membrane at a low level and thereby suppress the increase of the electrolysis voltage, it is necessary to increase the open area ratio of a woven fabric. However, if the open area ratio of a woven fabric is increased, misalignment tends to occur at an opening portion of the woven fabric, and therefore it is difficult to produce a woven fabric or insert it in an electrolyte membrane.
Accordingly, a method of employing a woven fabric obtained by mixedly weaving a thread of PTFE and a sacrificial thread (such as a thread of polyethylene terephthalate (hereinafter, referred to as PET)) which is soluble in an aqueous alkaline solution, has been proposed (Patent Documents 1 and 2).
A sacrificial thread is eluted in an alkaline aqueous solution and removed (i) when a reinforced precursor membrane having a precursor membrane containing a fluoropolymer having precursor groups for the ion exchange groups reinforced by a woven fabric, is immersed in an alkaline aqueous solution, whereby the precursor groups are hydrolyzed and converted into the ion exchange groups to produce a reinforced electrolyte membrane or (ii) when the reinforced electrolyte membrane is disposed in an electrolytic cell to carry out conditioning operation before regular operation of alkali chloride electrolysis, and therefore the sacrificial thread does not affect the membrane resistance at the time of the regular operation of the alkali chloride electrolysis using a reinforced electrolyte membrane.